Biological availability of trace elements

Subgroup on Metals of the Tri-Academj Committee on Acid Deposition, Acid Deposition Effects on Geochemical Cjcling and Biological Availability of Trace Elements, National Academy Press, Washington, D.C., 1985. 

This discussion examines the recent progress of nutritional trace element research and its implications for trace element analysis. Elements recently identified as essential are present in low concentrations for which analytical methods are not yet reliable. Biological availability of trace elements depends on chemical form and on interactions with other inorganic and organic constituents of the diet. Therefore, information on elemental species is required, in addition to quantitative data. Finally, the demonstration of essential functions of trace elements previously known only for their toxicity necessitates establishing safe ranges of intake, free from danger of chronic toxicity but sufficient to meet human needs. 

Considerable recent research has focused on the topic of chemical speciation in the environment. It is increasingly realised that the distribution, mobility and biological availability of chemical elements depend not simply on their concentrations but, critically, on the forms in which they occur in natural systems. Continuing developments in analytical chemistry have made speciation practicable even where analytes are present at trace levels (as is often the case in natural samples). 

IAEA (1985) [Muramai SU Y, Parr RM] Survey of currently available reference materials for use in connection with the determination of trace elements in biological and environmental materials. Report IAEA/RL/128, International Atomic Energy Agency, Vienna, Austria. 

Luoma, S. N. (1989). Can we determine the biological availability of sediment-bound trace elements Hydrobiologia, 176/177, 379-396. 

Several elements, particularly zinc and copper, could play a role as trace nutrients for phytoplankton. They are known to be important for growth of terrestrial plants, but neither the requirement for these nutrients nor the elemental distributions in seawater are well known. The biological availability of both zinc and copper is controlled by their complexation with organic material. Analytical methods that have the distinction of being able to discriminate chemical forms of the metal are needed. These measurements reflect the chemical reactivity and biological availability or toxicity of the metal more accurately. 

There is a danger of creating new imbalances by careless use of supplements of vitamins and trace minerals, hormones and drugs. Examples of interaction between trace elements and vitamins include the following vitamin C (ascorbic acid) enhances the biological availability of iron, de-... 

Jenne, E. A., and Luoma, S. N. The forms of trace elements in soils, sediment, and associated waters An overview of their determination and biological availability, p. 110-143, in Wildung, R. E., and Drucker, H., ed., "Biological Implications of Metals in the Environment." CONF-750929, NTIS Springfield, Virginia, 1977. 

Biological Implications of Chemical Forms. The biological availability of many trace elements is influenced by their valence state. Ferrous iron is believed to be more readily available than the ferric form, and selenium is better absorbed in its high oxidation state than in its lower ones. The organism is able to oxidize or reduce some, but not all, trace elements to their biologically active form. It is important, therefore, to determine the valence state in biological material, particularly in those cases where great differences of availability or toxicity exist, as in the case of chromium or of mercury. 

Similar demands for speciation of trace elements exist for food analysis. Substantial differences in the biological availability are known for several essential elements and depend on the form in which they are present in the diet. The chemical bases for these differences are known for cobalt, iron, and chromium but not for zinc, copper, and selenium. The importance of speciation in food analysis is best demonstrated by the example of iron. That element, when part of heme compounds, is well absorbed, and there is little influence on the absorption by other factors in the diet. Nonheme iron, on the other hand, is not readily absorbed and, in addition, is subject to many influences from dietary ingredients those influences are poorly understood and probably not completely known (14). 

Until now the attainment of these results was not possible, as few data are available to describe the trace metal concentrations in the Southern Ocean (5-11). In order to contribute to the knowledge of the distribution of trace elements and of the biological cycles in Antarctica over the past twelve Italian expeditions, attention was focused, among others, on trace metals in solid and in dissolved phases (12 29). 

Translocation in plant. The last step in elemental availability is translocation from roots to tops, a biological process that would seem outside the control of soil chemistry. A number of trace elements, especially those that take the form of cations in soils, do not readily translocate to plant tops. Therefore it is common for metals such as Cu, Pb, and Cd, after absorption from the soil, to accumulate in (or on) roots. Translocation behavior of plants is complex and not understood for many of the elements of environmental concern, but soil chemical factors (alkalinity, phosphate... 

While ranges of total concentration serve to set bounds for experimentally determining effects on marine populations, the actual species of metal ion available to the biological population is of importance. Sillen, in a classic paper, has computed the stable species of many metals in sea water21). He concluded, for example, that Hg+2, Cd+2, and Pb+2 exist primarily as chloride complexes. pH determines the availability of the hydroxide ion and thereby the solubility of metal hydroxides. Sillen assumed a pH of 8.1 0.2 as representative. Significant variations could occur, however, in estuarine waters. When concentrations of trace elements were compared with calculations of their solubility products and stability constants, the observed values were considerably less than the calculated values. The implication is that the heavy metals are not in equilibrium with solid phases of their salts, but that other processes, such as chelation and adsorption, control their concentration. 

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